Disruption of the A-C Membrane

Question 1

What processes/diseases can disrupt the A-C membrane?

Answer 1

Inflammation (e.g. diffuse alveolar damage, ARDS, SLE, RA)
Infection (i.e. pneumonitis or alveolitis, can be caused by TB)
Fibrosis
Emphysema
Fluid
Cancer

Question 2

What is the normal limiting factor of oxygen diffusion? What occurs when there is a gross abnormality of the A-C membrane?

Answer 2

Normally perfusion-limited (O2 diffusion is increased by increasing CO)
Diffusion limitation only occurs at rest if the A-C membrane is grossly abnormal (or with less severe disease during exercise)

Question 3

When does diffusion limitation of CO2 occur? Why?

Answer 3

Only with very severe abnormalities of the A-C membrane, AFTER O2 diffusion limitation (because CO2 diffusion is 20x faster)

Question 4

What are the 3 possible causes of a low PaO2?

Answer 4

Low PiO2 (e.g. in high altitudes)
Low ventilation
Abnormal gas exchange (e.g. low V/Q units, shunt or diffusion impairment)

Question 5

What does a wide A-a gradient indicate?

Answer 5

There is a gas exchange abnormality

Question 6

What causes a high PaCO2?

Answer 6

Low ventilation

Question 7

How do diseases of the A-C membrane increase WOB?

Answer 7

There is reduced lung compliance and therefore increased elastic WOB